Electric power can be distributed to customers through overhead lines, underground lines, or a combination of overhead and underground lines. Faults can occur in the power lines and cause a loss of power to customers. These faults can be a result of an open circuit or a short circuit (e.g., where two phases touch each other). As an example, when a power line has a short, the power supply will stop and leave many customers without power. In the case of the overhead lines, because the lines are visible, the detection of a problem can be easier, and in many instances customers will point the problem out.
However, with underground lines the problem of detection is much more complex. When a short occurs there is little or no indication at the surface, except on a rare occasion of a blown manhole. Accordingly, it can be difficult to identify the line or lines that have a fault. In fact, often the majority of the repair time is wasted on finding the location of the fault.
Often techniques such as reflectometry or time-delayed analysis are used to find the location of a fault. One of such techniques uses an application of a high voltage to an underground cable to break down the insulation defect and induce a transient fault pulse to propagate through the cable to the end. For example, one method for locating a fault that has occurred in an electric power cable in relationship to one or more known locations along the cable includes the steps of applying a high voltage of a predetermined polarity in the cable on one side of the fault to induce the transient fault pulse; sensing the transient fault pulse at two known locations on either side of the fault; determining the pulse direction at each sensing location; and if the directions are opposite recognizing that the fault is between the two sensing locations. Even techniques involving sensors tend to incorporate time delay or reflectometry methods. Moreover, methods involving RF transmitters to send the sensor signals to a central station can have electromagnetic interference from the power lines themselves.
Another common method of detecting a short circuit is to use a ground probe to search for maximum signal strength. Here, the maximum current point will be nearest to the fault. There are three basic methods that might be used to show the direction to fault: DC shift; cable carrier and/or locked carrier reference; and phase deviation. The DC shift method involves either placing a large DC voltage on the cable or using a large pseudo impulse. The method using cable carrier and/or locked carrier reference involves locking an on-board reference to the transmitter. This can be most easily achieved by sending a carrier signal down the same cable and picking it up with an antenna. Alternatively, a radio based carrier system could be used. Another method is to lock the receiver to the transmitter and hold the lock using a very low drift oscillator. The phase deviation method involves using more than one frequency and measuring the direction of phase deviation between the two signals. If the direction of deviation is one polarity, then the fault lies in one direction, otherwise it lies in the other. However, these methods require time to find the fault's location.
Therefore, there is a need for a method and fault detection system for detecting and identifying the location of faults in underground power lines that can effectively and quickly identify faults in underground power lines.